1. Field of the Invention
The present invention relates generally to an optical pickup device for use in an optical information recording/reproducing apparatus, and more particularly, to an optical pickup including a focus error detecting circuit using an astigmatism method.
2. Description of the Related Art
For a recording/reproducing apparatus for recording/reproducing information on an optical disc, loaded therein, such as an optical video disc, a digital audio disc, and so on, a focus servo and a tracking servo are essential for always accurately converging light beams for writing and reading information to a pit train or the like formed spirally or concentrically on a recording surface of the optical disc. The focus servo performs a positional control for an objective lens, used to irradiate a pit train on the optical disc with light beams, in an optical axis direction so as to reduce a focus error, i. e., an error of the position of the objective lens in the optical axis direction with respect to the focus position of the objective lens. The tracking servo performs a positional control for the position of the objective lens, used to irradiate a pit train on the optical disc with light beams, with respect to a recording track in a radial direction of the optical disc, so as to reduce a tracking error, i. e., an error of the objective lens with respect to the pit train recording track position.
FIG. 1 illustrates a conventional optical pickup device using the astigmatism method.
A laser beam from a semiconductor laser 1 is transformed into a parallel laser beam by a collimator lens 2, passes through a polarizing beam splitter 3, and is converged by an objective lens 4 toward an optical disc 5 to form a light spot onto a pit train on an information recording surface of the optical disc 5. Light reflected from the optical disc 5 is converged by the objective lens 4 and directed by a beam splitter 3 to a detecting lens 7. Converged light formed by the detecting lens 7 passes through a cylindrical lens 8, serving as an astigmatism generating element, to form a spot image near the center `O` of a light receiving surface of a quadrant photodetector 9 having four light receiving surface areas (elements) divided by two orthogonal line segments. The cylindrical lens 8 irradiates the quadrant photodetector 9 with a light spot SP in the shape of true circle as illustrated in FIG. 2A when the laser beam is converged on the recording surface of the optical disc 5 in focus, and an elliptic light spot SP, extending in an orthogonal direction of the elements as illustrated in FIG. 2B or 2C when the converged laser beam is out of focus on the recording surface of the optical disc 5 (FIG. 2B illustrates the light spot SP when the objective lens 4 is too far from the optical disc 5, while FIG. 2C illustrates the light spot SP when the objective lens 4 is too near the optical disc 5), thus generating so-called astigmatism.
The quadrant photodetector 9 opto-electrically transduces the light spot irradiated to the four light receiving surface areas into respective electric signals which are supplied to a focus error detecting circuit 12. The focus error detecting circuit 12 generates a focus error signal (FES) based on the electric signals supplied from the quadrant photodetector 9 and supplies the focus error signal to an actuator driver circuit 13. The actuator driver circuit 13 supplies a focusing driving signal to an actuator 15. The actuator 15 moves the objective lens 4 in response to the focusing driving signal in the optical axis direction.
The focus error detecting circuit 12, as illustrated in FIG. 3, is connected to the quadrant photodetector 9, where the quadrant photodetector 9 is composed of four detecting elements DET1 to DET4 in first to fourth quadrants which are located adjacent to each other with two orthogonal division lines L1 and L2 interposed therebetween and which are independent of each other. The quadrant photodetector 9 is positioned such that the division line L2 is in parallel with a tangential direction with respect to the extending direction of the recording track, and the other division line L1 is in parallel with the radial direction of the same. Respective opto-electrically transduced outputs from the elements DET1 and DET3, symmetric with respect to the center `O` of the light receiving surface of the quadrant photodetector 9, are added by an adder 22, while respective opto-electrically transduced outputs from the elements DET2 and DET4, also symmetric with respect to the center `O` of the light receiving surface, are added by an adder 21, and outputs from the respective adders 21 and 22 are supplied to a differential amplifier 23. The differential amplifier 23 calculates the difference between the supplied signals, and outputs a signal indicative of the difference therebetween as a focus error signal (FES).
As described above, in the conventional focus error detecting circuit 12, the outputs of the quadrant photodetector 9 are added by the adders 21 and 22, respectively, and the differential amplifier 23 calculates the difference between the outputs of the adders 21 and 22 to generate a focus error component. In this event, when the light beam is in focus, the light spot in the shape of true circle as illustrated in FIG. 2A is formed on the quadrant photodetector 9, where a spot intensity distribution is symmetric with respect to the center `O` of the light receiving surface of the quadrant photodetector 9, i. e., symmetric in the tangential direction and in the radial direction, so that the values resulting from the additions of the opto-electrically transduced outputs from the elements on the diagonals are equal to each other, with the focus error component being calculated to be "zero". On the other hand, when the light beam is out of focus, i. e., an elliptic light spot extending in a diagonal direction as illustrated in FIG. 2B or 2C is formed on the quadrant photodetector 9, so that the values resulting from the additions of the opto-electrically transduced outputs from the elements on the diagonals are different from each other. Thus, the focus error component output from the differential amplifier 23 exhibits a value corresponding to the focus error. Specifically, assuming that the references designated to the elements of the quadrant photodetector 9 represent the outputs thereof, the focus error signal FES is expressed by the following equation: EQU FES=(DET1+DET3)-(DET2+DET4)
However, it is actually difficult to perfectly remove astigmatism in the conventional optical pickup device utilizing the astigmatism method, although optical elements (including a laser diode as a light source) are designed so that any further other astigmatism does not occur. In addition, birefringence of the substrate of the optical disc generates astigmatism. For example, the birefringence in the disc substrate made of polycarbonate (PC) generates astigmatism in the direction extending at the angle of 45 degree with respect to a tangential (track) or radial direction. In this case, when the conventional optical pickup device detects a focus error signal FES from the optical disc having a lands and a groove formed on an information recording surface thereof, the FES includes a noise during the crossing of the light spot over the tracks. This is because the astigmatism is changed in response to the variation of optical path in the substrate cased by the difference in the focus position of the light beam between the land and groove.
Further more, there may be sometimes an offset of the optical axis of the light spot from the center `O` of the light receiving surface in the tangential direction due to an error associated with the manufacturing of the pickup as indicated in the broken line shown in FIG. 3. If an offset is present in the optical axis of the beam spot in the tangential direction or there are another astigmatism in the optics and birefringence in the disc substrate, then the focus error signal FES does include noise. Such a noise is hereinafter called an "FES noise".
Since a conventional CD player has employed an objective lens having a small numerical aperture NA and a large focus depth, the noise more or less included in the focus error signal (FES) has not caused any problem as a focus error. However, when information is read from a recently developed optical disc having pregrooves such as DVD-RAM or the like, an objective lens having a large numerical aperture and a small focus depth is employed for purposes of reading information, so that the noise included in the focus error signal exerts larger influences on the focus servo for the objective lens.
This may cause a focus servo system to fail to follow the noise in an optical pickup device adapted to read information from an optical disc having pregrooves, resulting in oscillation of a circuit associated with the focus servo system.